Reconfigurable all-optical multiplexers with simultaneous add-drop capability

ABSTRACT

The invention provides a novel wavelength-separating-routing (WSR) apparatus that uses a diffraction grating to separate a multi-wavelength optical signal by wavelength into multiple spectral channels, which are then focused onto an array of corresponding channel micromirrors. The channel micromirrors are individually controllable and continuously pivotable to reflect the spectral channels into multiple output ports. As such, the inventive WSR apparatus is capable of routing the spectral channels on a channel-by-channel basis and coupling any spectral channel into any one of the output ports. The WSR apparatus of the present invention may be further equipped with servo-control and spectral power-management capabilities, thereby maintaining the coupling efficiencies of the spectral channels into the output ports at desired values. The WSR apparatus of the present invention can be used to construct a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for WDM optical networking applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-part of U.S. patent applicationSer. No. 09/938,426, filed on Aug. 23, 2001, which is incorporatedherein by reference in its entirety, and which claims priority from U.S.Provisional Patent Application No. 60/277,217, filed on Mar. 19, 2001.

FIELD OF THE INVENTION

This invention relates generally to optical communication systems. Morespecifically, it relates to a novel class of dynamically reconfigurableoptical add-drop multiplexers (OADMs) for wavelength divisionmultiplexed optical networking applications.

BACKGROUND

As fiber-optic communication networks rapidly spread into every walk ofmodern life, there is a growing demand for optical components andsubsystems that enable the fiber-optic communications networks to beincreasingly scalable, versatile, robust and cost-effective.

Contemporary fiber-optic communications networks commonly employwavelength division multiplexing (WDM), for it allows multipleinformation (or data) channels to be simultaneously transmitted on asingle optical fiber by using different wavelengths and therebysignificantly enhances the information bandwidth of the fiber. Theprevalence of WDM technology has made optical add-drop multiplexersindispensable building blocks of modem fiber-optic communicationnetworks. An optical add-drop multiplexer (OADM) serves to selectivelyremove (or drop) one or more wavelengths from a multiplicity ofwavelengths on an optical fiber, hence taking away one or more datachannels from the traffic stream on the fiber. It further adds one ormore wavelengths back onto the fiber, thereby inserting new datachannels in the same stream of traffic. As such, an OADM makes itpossible to launch and retrieve multiple data channels onto and from anoptical fiber respectively, without disrupting the overall traffic flowalong the fiber. Indeed, careful placement of the OADMs can dramaticallyimprove an optical communication network's flexibility and robustness,while providing significant cost advantages.

Conventional OADMs in the art typically employmultiplexers/demultiplexers (e.g. waveguide grating routers orarrayed-waveguide gratings), tunable filters, optical switches, andoptical circulators in a parallel or serial architecture to accomplishthe add and drop functions. In the parallel architecture, as exemplifiedin U.S. Pat. No. 5,974,207, a demultiplexer (e.g., a waveguide gratingrouter) first separates a multi-wavelength signal into its constituentspectral components. A wavelength switching/routing means (e.g., acombination of optical switches and optical circulators) then serves todrop selective wavelengths and add others. Finally, a multiplexercombines the remaining (i.e., the pass-through) wavelengths into anoutput multi-wavelength optical signal. In the serial architecture, asexemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g., Braggfiber gratings) in combination with optical circulators are used toseparate the drop wavelengths from the pass-through wavelengths andsubsequently launch the add channels into the pass-through path. And ifmultiple wavelengths are to be added and dropped, additionalmultiplexers and demultiplexers are required to demultiplex the dropwavelengths and multiplex the add wavelengths, respectively.Irrespective of the underlying architecture, the OADMs currently in theart are characteristically high in cost, and prone to significantoptical loss accumulation. Moreover, the designs of these OADMs are suchthat it is inherently difficult to reconfigure them in a dynamicfashion.

U.S. Pat. No. 6,204,946 to Askyuk et al. discloses an OADM that makes usof free-space optics in a parallel construction. In this case, amulti-wavelength optical signal emerging from an input port is incidentonto a ruled diffraction grating. The constituent spectral channels thusseparated are then focused by a focusing lens onto a linear array ofbinary micromachined mirrors. Each micromirror is configured to operatebetween two discrete states, such that it either retroreflects itscorresponding spectral channel back into the input port as apass-through channel, or directs its spectral channel to an output portas a drop channel. As such, the pass-through signal (i.e., the combinedpass-through channels) shares the same input port as the input signal.An optical circulator is therefore coupled to the input port, to providenecessary routing of these two signals. Likewise, the drop channelsshare the output port with the add channels. An additional opticalcirculator is thereby coupled to the output port, from which the dropchannels exit and the add channels are introduced into the output port.The add channels are subsequently combined with the pass-through signalby way of the diffraction grating and the binary micromirrors.

Although the aforementioned OADM disclosed by Askyuk et al. has theadvantage of performing wavelength separating and routing in free spaceand thereby incurring less optical loss, it suffers a number oflimitations. First, it requires that the pass-through signal share thesame port/fiber as the input signal. An optical circulator therefore hasto be implemented, to provide necessary routing of these two signals.Likewise, all the add and drop channels enter and leave the OADM throughthe same output port, hence the need for another optical circulator.Moreover, additional means must be provided to multiplex the addchannels before entering the system and to demultiplex the drop channelsafter exiting the system. This additional multiplexing/demultiplexingrequirement adds more cost and complexity that can restrict theversatility of the OADM thus-constructed. Second, the opticalcirculators implemented in this OADM for various routing purposesintroduce additional optical losses, which can accumulate to asubstantial amount. Third, the constituent optical components must be ina precise alignment, in order for the system to achieve its intendedpurpose. There are, however, no provisions provided for maintaining therequisite alignment; and no mechanisms implemented for overcomingdegradation in the alignment owing to environmental effects such asthermal and mechanical disturbances over the course of operation.

U.S. Pat. No. 5,960,133 to Tomlinson discloses an OADM that makes use ofa design similar to that of Askyuk et al. In one embodiment, there areinput, output, drop and add beams arranged in a rectangular array. Eachmicromirror, being switchable between two discrete positions, eitherreflects its corresponding wavelength component from the input beam backto the output beam, or concomitantly reflects the wavelength componentfrom the input beam back to the drop beam and the same wavelengthcomponent from the add beam back to the output beam. Alternativeembodiment are also shown, where multiple add and drop beams, along withinput and output beams, are arranged in a two-dimensional array.However, as in the case of Askyuk et al., there are no provisionsprovided for maintaining requisite optical alignment in the system, andno mechanisms implemented for mitigating degradation in the alignmentdue to environmental effects over the course of operation. Moreover, itmay be difficult in some applications to align multiple optical beams ina two-dimensional configuration and further maintain the requisitealignment.

As such, the prevailing drawbacks suffered by the OADMs currently in theart are summarized as follows:

1) The wavelength routing is intrinsically static, rendering itdifficult to dynamically reconfigure these OADMs.

2) Add and/or drop channels often need to be multiplexed and/ordemultiplexed, thereby imposing additional complexity and cost.

3) Stringent fabrication tolerance and painstaking optical alignment arerequired. Moreover, the optical alignment is not actively maintained,rendering it susceptible to environmental effects such as thermal andmechanical disturbances over the course of operation.

4) In an optical communication network, OADMs are typically in a ring orcascaded configuration. In order to mitigate the interference amongstOADMs, which often adversely affects the overall performance of thenetwork, it is essential that the optical power levels of spectralchannels entering and exiting each OADM be managed in a systematic way,for instance, by introducing power (or gain) equalization at each stage.Such a power equalization capability is also needed for compensating fornonuniform gain caused by optical amplifiers (e.g., erbium doped fiberamplifiers) in the network. There lacks, however, a systematic anddynamic management of the optical power levels of various spectralchannels in these OASMs.

5) The inherent high cost and heavy optical loss further impede the wideapplication of these OADMs.

In view of the foregoing, there is an urgent need in the art for opticaladd-drop multiplexers that overcome the aforementioned shortcomings in asimple, effective, and economical construction.

SUMMARY OF THE INVENTION

The present invention provides a wavelength-separating-routing (WSR)apparatus and method which employ an array of fiber collimators servingas an input port and a plurality of output ports; awavelength-separator; a beam-focuser; and an array of channelmicromirrors.

In operation, a multi-wavelength optical signal emerges from the inputport. The wavelength separator separates the multi-wavelength opticalsignal into multiple spectral channels, each characterized by a distinctcenter wavelength and associated bandwidth. The beam-focuser focuses thespectral channels into corresponding focused spots. The channelmicromirrors are positioned such that each channel micromirror receivesone of the spectral channels. The channel micromirrors are individuallycontrollable and movable, e.g., continuously pivotable (or rotatable),so as to reflect the spectral channels into selected ones of the outputports. As such, each channel micromirror is assigned to a specificspectral channel, hence the name “channel micromirror”. Each output portmay receive any number of the reflected spectral channels.

A distinct feature of the channel micromirrors in the present invention,in contrast to those used in the prior art, is that the pivoting (orrotational) motion of each channel micromirror is under analog controlsuch that its pivoting angle can be continuously adjusted. This enableseach channel micromirror to scan its corresponding spectral channelacross all possible output ports and thereby direct the spectral channelto any desired output port.

In the WSR apparatus of the present invention, the wavelength-separatormay be provided by a ruled diffraction grating, a holographicdiffraction grating, an echelle grating, a curved diffraction grating, atransmission grating, a dispersing prism, or other types ofwavelength-separating means known in the art. The beam-focuser may be asingle lens, an assembly of lenses, or other types of beam-focusingmeans known in the art. The channel micromirrors may be provided bysilicon micromachined mirrors, reflective ribbons (or membranes), orother types of beam-deflecting means known in the art. Each channelmicromirror may be pivotable about one or two axes. The fibercollimators serving as the input and output ports may be arranged in aone-dimensional or two-dimensional array. In the latter case, thechannel micromirrors must be pivotable biaxially.

The WSR apparatus of the present invention may further comprise an arrayof collimator-alignment mirrors, in optical communication with thewavelength-separator and the fiber collimators, for adjusting thealignment of the input multi-wavelength signal and directing thereflected spectral channels into the selected output ports by way ofangular control of the collimated beams. Each collimator-alignmentmirror may be rotatable about one or two axes. The collimator-alignmentmirrors may be arranged in a one-dimensional or two-dimensional array.First and second arrays of imaging lenses may additionally be opticallyinterposed between the collimator-alignment mirrors and the fibercollimators in a telecentric arrangement, thereby “imaging” thecollimator-alignment mirrors onto the corresponding fiber collimators toensure an optimal alignment.

The WSR apparatus of the present invention may further include aservo-control assembly, in communication with the channel micromirrorsand the output ports. The servo-control assembly serves to monitor theoptical power levels of the spectral channels coupled into the outputports and further provide control of the channel micromirrors on anindividual basis, so as to maintain a predetermined coupling efficiencyfor each spectral channel into an output port. (If the WSR apparatusincludes an array of collimator-alignment mirrors as described above,the servo-control assembly may additionally provide dynamic control ofthe collimator-alignment mirrors.) As such, the servo-control assemblyprovides dynamic control of the coupling of the spectral channels intothe respective output ports and actively manages the optical powerlevels of the spectral channels coupled into the output ports. Forexample, the optical power levels of the spectral channels coupled intothe output ports can be equalized at a predetermined value. Moreover,the utilization of such a servo-control assembly effectively relaxes thefabrication tolerances and precision during assembly of a WSR apparatusof the present invention, and further enables the system to correct forshift in optical alignment that may arise over the course of operation.A WSR apparatus incorporating a servo-control assembly thus described istermed a WSR-S apparatus, in the following discussion.

Accordingly, the WRS-S (or WRS) apparatus of the present invention maybe used to construct a variety of optical devices, including a novelclass of dynamically reconfigurable optical add-drop multiplexers(OADMs).

In one embodiment of an OADM according to the present invention, aone-dimensional input-output-port array, including an input port, apass-through port, a plurality of drop ports, and a plurality of addports, may be implemented in a WSR of the present invention. Thearrangement of the input-output-port array may be such that the inputports (i.e., the input port and the add ports) transmitting the incomingoptical signals and the output ports (i.e., the pass-through and thedrop ports) carrying the outgoing optical signal are positioned in analternating (or interleaved) fashion, whereby interposed between everytwo input ports is an output port, and vice versa. Such an arrangementwarrants that if a spectral channel originating from the input port isto be routed to a drop port, an add spectral channel with the samewavelength from an adjacent (or pairing) add port can be simultaneouslydirected into the pass-through port. This is due to the fact that thedrop spectral channel and the corresponding add spectral channel arerouted to their respective destination by the same channel micromirrorin the WSR apparatus.

In an alternative embodiment of an OADM according to the presentinvention, a two-dimensional input-output-port array may be implementedin a WSR apparatus of the present invention. The input-output-port arraycomprises an input-port column having an input port and a plurality ofadd ports, and an output-port column including a pass-through port and aplurality of drop ports. In this arrangement, each input port forms a“pair” with its adjacent output port, thereby enabling each channelmicromirror to route a spectral channel from the input port to a dropport and simultaneously direct an add spectral channel (with the samewavelength) from a pairing add port to the pass-through port.

As such, a notable advantage of the aforementioned OADMs is the abilityto add and drop multiple spectral channels in a dynamicallyreconfigurable fashion, without involving additional components such asoptical circulators and/or optical combiners. A servo-control assemblymay be further incorporated in an OADM of the present invention, formonitoring and controlling the optical power levels of the spectralchannels coupled into the output ports.

The OADMs of the present invention provide many advantages over theprior devices, notably:

1) By advantageously employing an array of channel micromirrors that areindividually and continuously controllable, an OADM of the presentinvention is capable of routing the spectral channels on achannel-by-channel basis and directing any spectral channel into any oneof multiple output ports. As such, its underlying operation isdynamically reconfigurable, and its underlying architecture isintrinsically scalable to a large number of channel counts.

2) The add and drop spectral channels need not be multiplexed anddemultiplexed before entering and after leaving the OADM respectively.And there are not fundamental restrictions on the wavelengths to beadded or dropped.

3) The coupling of the spectral channels into the output ports isdynamically controlled by a servo-control assembly, rendering the OADMless susceptible to environmental effects (such as thermal andmechanical disturbances) and therefore more robust in performance. Bymaintaining an optimal optical alignment, the optical losses incurred bythe spectral channels are also significantly reduced.

4) The optical power levels of the spectral channels coupled into thepass-through port can be dynamically managed according to demand, ormaintained at desired values (e.g., equalized at a predetermined value)by way of the servo-control assembly. The optical power levels of thespectral channels coupled into the drop ports can also be monitored bythe servo-control assembly. This spectral power-management capability asan integral part of the OADM will be particularly desirable in WDMoptical networking applications.

5) The use of free-space optics provides a simple, low loss, andcost-effective construction. Moreover, the utilization of theservo-control assembly effectively relaxes the fabrication tolerancesand precision during initial assembly, enabling the OADM to be simplerand more adaptable in structure, lower in cost and optical loss.

6) The underlying OADM architecture allows a multiplicity of the OADMsaccording to the present invention to be readily assembled (e.g.,cascaded) for WDM optical networking applications.

The novel features of this invention, as well as the invention itself,will be best understood from the following drawings and detaileddescription.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1D show a first embodiment of a wavelength-separating-routing(WSR) apparatus according to the present invention, and the modelingresults demonstrating the performance of the WSR apparatus;

FIG. 2A depicts a second embodiment of a WSR apparatus according to thepresent invention;

FIGS. 2B-2C show a third embodiment of a WSR apparatus according to thepresent invention;

FIG. 3 depicts a fourth embodiment of a WSR apparatus according to thepresent invention;

FIGS. 4A-4B show schematic illustrations of two embodiments of a WSR-Sapparatus comprising a WSR apparatus and a servo-control assembly,according to the present invention;

FIGS. 5A-5B depicts a first embodiment of an optical add-dropmultiplexer (OADM) according to the present invention;

FIGS. 6A-6B shows a second embodiment of an OADM according to thepresent invention; and

FIG. 7 shows an exemplary embodiment of an OADM employing aservo-control assembly, according to the present invention.

DETAILED DESCRIPTION

In this specification and appending claims, a “spectral channel” ischaracterized by a distinct center wavelength and associated bandwidth.Each spectral channel may carry a unique information signal, as in WDMoptical networking applications.

FIG. 1A depicts a first embodiment of a wavelength-separating-routing(WSR) apparatus according to the present invention. By way of example toillustrate the general principles and the topological structure of awavelength-separating-routing (WSR) apparatus of the present invention,the WSR apparatus 100 comprises multiple input/output ports which may bein the form of an array of fiber collimators 110, providing an inputport 110-1 and a plurality of output ports 110-2 through 110-N (N≧3); awavelength-separator which in one form may be a diffraction grating 101;a beam-focuser in the form of a focusing lens 102; and an array ofchannel micromirrors 103.

In operation, a multi-wavelength optical signal emerges from the input110-1. The diffraction grating 101 angularly separates themulti-wavelength optical signal into multiple spectral channels, whichare in turn focused by the focusing lens 102 into a spatial array ofcorresponding focused spots (not shown in FIG. 1A). The channelmicromirrors 103 are positioned in accordance with the spatial arrayformed by the spectral channels, such that each channel micromirrorreceives one of the spectral channels. The channel micromirrors 103 areindividually controllable and movable, e.g., pivotable (or rotatable)under analog (or continuous) control, such that, upon reflection, thespectral channels are directed into selected ones of the output ports110-2 through 110-N by way of the focusing lens 102 and the diffractiongrating 101. As such, each channel micromirror is assigned to a specificspectral channel, hence the name “channel micromirror”. Each output portmay receive any number of the reflected spectral channels.

For purpose of illustration and clarity, only a select few (e.g., three)of the spectral channels, along with the input multi-wavelength opticalsignal, are graphically illustrated in FIG. 1A and the followingfigures. It should be noted, however, that there can be any number ofthe spectral channels in a WSR apparatus of the present invention (solong as the number of spectral channels does not exceed the number ofchannel mirrors employed in the system). It should also be noted thatthe optical beams representing the spectral channels shown in FIG. 1Aand the following figures are provided for illustrative purpose only.That is, their sizes and shapes may not be drawn according to scale. Forinstance, the input beam and the corresponding diffracted beamsgenerally have different cross-sectional shapes, so long as the angle ofincidence upon the diffraction grating is not equal to the angle ofdiffraction, as is known to those skilled in the art.

In the embodiment of FIG. 1A, it is preferable that the diffractiongrating 101 and the channel micromirrors 103 are placed respectively inthe first and second (i.e., the front and back) focal planes (on theopposing sides) of the focusing lens 102. Such a telecentric arrangementallows the chief rays of the focused beams to be parallel to each otherand generally parallel to the optical axis. In this application, thetelecentric configuration further allows the reflected spectral channelsto be efficiently coupled into the respective output ports, therebyminimizing various translational walk-off effects that may otherwisearise. Moreover, the multi-wavelength input optical signal is preferablycollimated and circular in cross-section. The corresponding spectralchannels diffracted from the diffraction grating 101 are generallyelliptical in cross-section; they may be of the same size as the inputbeam in one dimension and elongated in the other dimension.

It is known that the diffraction efficiency of a diffraction grating isgenerally polarization-dependent. For instance, the diffractionefficiency of a grating in a standard mounting configuration may beconsiderably higher for p (or TM) polarization (perpendicular to thegroove lines on the grating) than for s (or TE) polarization (orthogonalto p-polarization), or vice versa. To mitigate suchpolarization-sensitive effects, a quarter-wave plate 104 may beoptically interposed between the diffraction grating 101 and the channelmicromirrors 103, and preferably placed between the diffraction grating101 and the focusing lens 102 as is shown in FIG. 1A. In this way, eachspectral channel experiences a total of approximately 90-degree rotationin polarization upon traversing the quarter-wave plate 104 twice. (Thatis, if an optical beam has p-polarization when first encountering thediffraction grating, it would have predominantly (if not all)s-polarization upon the second encountering, and vice versa.) Thisensures that all the spectral channels incur nearly the same amount ofround-trip polarization dependent loss.

In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101, by wayof example, is oriented such that the focused spots of the spectralchannels fall onto the channel micromirrors 103 in a horizontal array,as illustrated in FIG. 1B.

Depicted in FIG. 1B is a close-up view of the channel micromirrors 103shown in the embodiment of FIG. 1A. By way of example, the channelmicromirrors 103 are arranged in a one-dimensional array along thex-axis (i.e., the horizontal direction in the figure), so as to receivethe focused spots of the spatially separated spectral channels in aone-to-one correspondence. (As in the case of FIG. 1A, only threespectral channels are illustrated, each represented by a convergingbeam.) The reflective surface of each channel micromirror lies in thex-y plane as defined in the figure and be movable, e.g., pivotable (ordeflectable) about an axis along the x-direction in an analog (orcontinuous) manner. Each spectral channel, upon reflection, is deflectedin the y-direction (e.g., downward) relative to its incident direction,so as to be directed into one of the output ports 110-2 through 110-Nshown in FIG. 1A.

As described above, a unique feature of the present invention is thatthe motion of each channel micromirror is individually and continuouslycontrollable, such that its position, e.g., pivoting angle, can becontinuously adjusted. This enables each channel micromirror to scan itscorresponding spectral channel across all possible output ports andthereby direct the spectral channel to any desired output port. Toillustrate this capability, FIG. 1C shows a plot of coupling efficiencyas a function of a channel micromirror's pivoting angle θ, provided by aray-tracing model of a WSR apparatus in the embodiment of FIG. 1A. Asused herein, the coupling efficiency for a spectral channel is definedas the ratio of the amount of optical power coupled into the fiber corein an output port to the total amount of optical power incident upon theentrance surface of the fiber (associated with the fiber collimatorserving as the output port). In the ray-tracing model, the input opticalsignal is incident upon a diffraction grating with 700 lines permillimeter at a grazing angle of 85 degrees, where the grating is blazedto optimize the diffraction efficiency for the “−1” order. The focusinglens has a focal length of 100 mm. Each output port is provided by aquarter-pitch GRIN lens (2 mm in diameter) coupled to an optical fiber(see FIG. 1D). As displayed in FIG. 1C, the coupling efficiency varieswith the pivoting angle θ, and it requires about a 0.2-degree change inθ for the coupling efficiency to become practically negligible in thisexemplary case. As such, each spectral channel may practically acquireany coupling efficiency value by way of controlling the pivoting angleof its corresponding channel micromirror. This is also to say thatvariable optical attenuation at the granularity of a single wavelengthcan be obtained in a WSR apparatus of the present invention. FIG. 1Dprovides ray-tracing illustrations of two extreme points on the couplingefficiency vs. θ curve of FIG. 1C: on-axis coupling corresponding toθ=0, where the coupling efficiency is maximum; and off-axis couplingcorresponding to θ=0.2 degrees, where the representative collimated beam(representing an exemplary spectral channel) undergoes a significanttranslational walk-off and renders the coupling efficiency practicallynegligible. The exemplary modeling results thus described demonstratethe unique capabilities of the WSR apparatus of the present invention.

FIG. 1A provides one of many embodiments of a WSR apparatus according tothe present invention. In general, the wavelength-separator is awavelength-separating means that may be a ruled diffraction grating, aholographic diffraction grating, an echelle grating, a dispersing prism,or other types of spectral-separating means known in the art. Thebeam-focuser may be a focusing lens, an assembly of lenses, or otherbeam-focusing means known in the art. The focusing function may also beaccomplished by using a curved diffraction grating as thewavelength-separator. The channel micromirrors may be provided bysilicon micromachined mirrors, reflective ribbons (or membranes), orother types of beam-deflecting elements known in the art. Eachmicromirror may be pivoted about one or two axes. It is important thatthe pivoting (or rotational) motion of each channel micromirror beindividually controllable in an analog manner, whereby the pivotingangle can be continuously adjusted so as to enable the channelmicromirror to scan a spectral channel across all possible output ports.The underlying fabrication techniques for micromachined mirrors andassociated actuation mechanisms are well documented in the art, see U.S.Pat. No. 5,629,790 for example. Moreover, a fiber collimator istypically in the form of a collimating lens (such as a GRIN lens) and aferrule-mounted fiber packaged together in a mechanically rigidstainless steel (or glass) tube. The fiber collimators serving as theinput and output ports may be arranged in a one-dimensional array, atwo-dimensional array, or other desired spatial pattern. For instance,they may be conveniently mounted in a linear array along a V-groovefabricated on a substrate made of silicon, plastic, or ceramic, ascommonly practiced in the art. It should be noted, however, that theinput port and the output ports need not necessarily be in close spatialproximity with each other, such as in an array configuration (although aclose packing would reduce the rotational range required for eachchannel micromirror). Those skilled in the art will know how to design aWSR apparatus according to the present invention, to best suit a givenapplication.

A WSR apparatus of the present invention may further comprise an arrayof collimator-alignment mirrors, for adjusting the alignment of theinput multi-wavelength optical signal and facilitating the coupling ofthe spectral channels into the respective output ports, as shown inFIGS. 2A-2B and 3.

Depicted in FIG. 2A is a second embodiment of a WSR apparatus accordingto the present invention. By way of example, WSR apparatus 200 is builtupon and hence shares a number of the elements used in the embodiment ofFIG. 1A, as identified by those elements labeled with identicalnumerals. Moreover, a one-dimensional array 220 of collimator-alignmentmirrors 220-1 through 220-N is optically interposed between thediffraction grating 101 and the fiber collimator array 110. Thecollimator-alignment mirror 220-1 is designated to correspond with theinput port 110-1, for adjusting the alignment of the multi-wavelengthinput optical signal and therefore ensuring that the spectral channelsimpinge onto the corresponding channel micromirrors. Thecollimator-alignment mirrors 220-2 through 220-N are designated to theoutput ports 110-2 through 110-N in a one-to-one correspondence, servingto provide angular control of the collimated beams of the reflectedspectral channels and thereby facilitating the coupling of the spectralchannels into the respective output ports according to desired couplingefficiencies. Each collimator-alignment mirror may be rotatable aboutone axis, or two-axes.

The embodiment of FIG. 2A is attractive in applications where the fibercollimators (serving as the input and output ports) are desired to beplaced in close proximity to the collimator alignment mirror array 220.To best facilitated the coupling of the spectral channels into theoutput ports, imaging lenses may be implemented between thecollimator-alignment mirror array 220 and the fiber collimator array110, as depicted in FIG. 2B. By way of example, WSR apparatus 250 ofFIG. 2B is built upon and hence shares many of the elements used in theembodiment of FIG. 2A, as identified by those elements labeled withidentical numerals. Additionally, first and second arrays 260, 270 ofimaging lenses are placed in a 4-f telecentric arrangement with respectto the collimator-alignment mirror array 220 and the fiber collimatorarray 110. The dashed box 280 shown in FIG. 2C provides a top view ofsuch a telecentric arrangement. In this case, the imaging lenses in thefirst and second arrays 260, 270 all have the same focal length f. Thecollimator-alignment mirrors 220-1 through 220-N are placed at therespective first (or front) focal points of the imaging lenses in thefirst array 260. Likewise, the fiber collimators 110-1 through 110-N areplaced at the respective second (or back) focal points of the imaginglenses in the second array 270. The separation between the first andsecond arrays 260, 270 of imaging lenses is 2 f. In this way, thecollimator-alignment mirrors 220-1 through 220-N are effectively imagedonto the respective entrance surfaces (i.e., the front focal planes) ofthe GRIN lenses in the corresponding fiber collimators 110-1 through110-N. Such a 4-f imaging system substantially eliminates translationalwalk-off of the collimated beams at the output ports that may otherwiseoccur as the mirror angles change.

FIG. 3 shows a fourth embodiment of a WSR apparatus according to thepresent invention. By way of example, WSR apparatus 300 is built uponand hence shares a number of the elements used in the embodiment of FIG.2B, as identified by those elements labeled with identical numerals. Inthis case, the one-dimensional fiber collimator array 110 of FIG. 2B isreplaced by a two-dimensional array 350 of fiber collimator, providingfor an input-port and a plurality of output ports. Accordingly, theone-dimensional collimator-alignment mirror array 220 of FIG. 2B isreplaced by a two-dimensional array 320 of collimator-alignment mirrors,and first and second one-dimensional arrays 260, 270 of imaging lensesof FIG. 2B are likewise replaced by first and second two-dimensionalarrays 360, 370 of imaging lenses, respectively. As in the case of theembodiment of FIG. 2B, the first and second two-dimensional arrays 360,370 of imaging lenses are placed in a 4-f telecentric arrangement withrespect to the two-dimensional collimator-alignment mirror array 320 andthe two-dimensional fiber collimator array 350. Each of the channelmicromirrors 103 must be pivotable biaxially in this case, in order todirect its corresponding spectral channel to any one of the outputports. As such, the WSR apparatus 300 is equipped to support a greaternumber of output ports.

In addition to facilitating the coupling of the spectral channels intothe respective output ports as described above, the collimator-alignmentmirrors in the above embodiments also serve to compensate formisalignment (e.g., due to fabrication and assembly errors) in the fibercollimators that provide for the input and output ports. For instance,relative misalignment between the fiber cores and their respectivecollimating lenses in the fiber collimators can lead to pointing errorsin the collimated beams, which may be corrected for by thecollimator-alignment mirrors. For these reasons, thecollimator-alignment mirrors are preferably rotatable about two axes.They may be silicon micromachined mirrors, for fast rotational speeds.They may also be other types of mirrors or beam-deflecting elementsknown in the art.

To optimize the coupling of the spectral channels into the output portsand further maintain the optimal optical alignment against environmentaleffects such as temperature variations and mechanical instabilities overthe course of operation, a WSR apparatus of the present invention mayincorporate a servo-control assembly, for providing dynamic control ofthe coupling of the spectral channels into the respective output portson a channel-by-channel basis. A WSR apparatus incorporating aservo-control assembly is termed a WSR-S apparatus in thisspecification.

FIG. 4A depicts a schematic illustration of a first embodiment of aWSR-S apparatus according to the present invention. The WSR-S apparatus400 comprises a WSR apparatus 410 and a servo-control assembly 440. TheWSR apparatus 410 may be substantially similar to the WSR apparatus 100of FIG. 1A, or any other embodiment in accordance with the presentinvention. The servo-control assembly 440 includes a spectral powermonitor 460, for monitoring the optical power levels of the spectralchannels coupled into the output ports 420-1 through 420-N of the WSRapparatus 410. By way of example, the spectral power monitor 460 may becoupled to the output ports 420-1 through 420-N by way of fiber-opticcouplers 420-1-C through 420-N-C, wherein each fiber-optic couplerserves to “tap off” a predetermined fraction of the optical signal inthe corresponding output port. The servo-control assembly 440 furtherincludes a processing unit 470, in communication with the spectral powermonitor 460 and the channel micromirrors 430 of the WSR apparatus 410.The processing unit 470 uses the optical power measurements from thespectral power monitor 460 to provide feedback control of the channelmicromirrors 430 on an individual basis, so as to maintain a desiredcoupling efficiency for each spectral channel into a selected outputport.

By way of example, the processing unit 470 may apply an appropriatealternating (or “dither”) control signal to a channel micromirror, insuperposition with the dc control signal for maintaining the channelmicromirror at a particular pivoting position. This enables both theoptical power level of the corresponding spectral channel and the rateof change in the optical power level (or the time derivative of theoptical power level) at the instant micromirror's pivoting angle to beobtained. In view of the exemplary coupling efficiency curve depicted inFIG. 1C, the rate of change in the optical power level is proportionalto the slope of the coupling efficiency curve, and is therefore usefulin locating the micromirror's pivoting angle corresponding to themeasured optical power level. It is also useful in determining themagnitude of the feedback control signal to be applied to the channelmicromirror, so as to achieve the desired coupling efficiency in a mosteffective manner. From the teachings of the present invention, a skilledartisan will know how to devise an appropriate servo control scheme, tobest suit a given application.

As such, the servo-control assembly 440 provides dynamic control of thecoupling of the spectral channels into the respective output ports on achannel-by-channel basis and thereby manages the optical power levels ofthe spectral channels coupled into the output ports. The optical powerlevels of the spectral channels in the output ports may be dynamicallymanaged according to demand, or maintained at desired values (e.g.,equalized at a predetermined value) in the present invention. Such aspectral power-management capability is essential in WDM opticalnetworking applications, as discussing above.

FIG. 4B depicts a schematic illustration of a second embodiment of aWSR-S apparatus according to the present invention. The WSR-S apparatus450 comprises a WSR apparatus 480 and a servo-control assembly 490. Inaddition to the channel micromirrors 430 (and other elements identifiedby the same numerals as those used in FIG. 4A), the WSR apparatus 480further includes a plurality of collimator-alignment mirrors 485, andmay be configured according to the embodiment of FIG. 2A, 2B, 3, or anyother embodiment in accordance with the present invention. By way ofexample, the servo-control assembly 490 includes the spectral powermonitor 460 as described in the embodiment of FIG. 4A, and a processingunit 495. In this case, the processing unit 495 is in communication withthe channel micromirrors 430 and the collimator-alignment mirrors 485 ofthe WSR apparatus 480, as well as the spectral power monitor 460. Theprocessing unit 495 uses the optical power measurements from thespectral power monitor 460 to provide dynamic control of the channelmicromirrors 430 along with the collimator-alignment mirrors 485, so asto maintain the coupling efficiencies of the spectral channels into theoutput ports at desired values. The underlying operating principle ofthe processing unit 495 may be substantially similar to that ofprocessing unit 470, as described above.

In the embodiment of FIG. 4A or 4B, the spectral power monitor 460 maybe one of spectral power monitoring devices known in the art that iscapable of detecting the optical power levels of spectral components ina multi-wavelength optical signal. Such devices are typically in theform of a wavelength-separating means (e.g., a diffraction grating) thatspatially separates a multi-wavelength optical signal by wavelength intoconstituent spectral components, and one or more optical sensors (e.g.,an array of photodiodes) that are configured such to detect the opticalpower levels of these spectral components. The processing unit 470 inFIG. 4A (or the processing unit 495 in FIG. 4B) typically includeselectrical circuits and signal processing programs for processing theoptical power measurements received from the spectral power monitor 460and generating appropriate control signals to be applied to the channelmicromirrors 430 (and the collimator-alignment mirrors 485 in the caseof FIG. 4B), so as to maintain the coupling efficiencies of the spectralchannels into the output ports at desired values. The electroniccircuitry and the associated signal processing algorithm/software forsuch a processing unit in a servo-control system are known in the art.Those skilled in the art will know how to implement a suitable spectralpower monitor along with an appropriate processing unit to provide aservo-control assembly in a WSP-S apparatus according to the presentinvention, for a given application.

The incorporation of a servo-control assembly provides additionaladvantages of effectively relaxing the requisite fabrication tolerancesand the precision of optical alignment during initial assembly of a WSRapparatus of the present invention, and further enabling the system tocorrect for shift in the alignment that may arise over the course ofoperation. By maintaining an optimal optical alignment, the opticallosses incurred by the spectral channels are also significantly reduced.As such, the WSR-S apparatus thus constructed is simpler and moreadaptable in structure, more robust in performance, and lower in costand optical loss. Accordingly, the WSR-S (or WSR) apparatus of thepresent invention may be used to construct a variety of optical devices,including a novel class of optical add-drop multiplexers (OADMs) for WDMoptical networking applications.

FIGS. 5A-5B depict a first embodiment of an optical add-drop multiplexer(OADM) of the present invention. Shown in FIG. 5A is a schematic view ofOADM 500 according to the present invention, which only illustrates theinput and output ports of WSR apparatus, along with a plurality of addports. The remainder of the WSR apparatus is schematically representedby a dashed box 510, for purpose of simplicity and clarity. The WSRapparatus in this case may be constructed according to the embodiment ofFIG. 1A, 2A, 2B, 3, or any other configuration in accordance with thepresent invention. By way of example, an input port 520, a pass-throughport 530, and a plurality of drop ports 540-1 through 540-N may bearranged, along with a plurality of add ports 560-1 through 560-N, in aone-dimensional input-output-port array 570. The arrangement of theinput-output-port array 570 may be such that the input ports (e.g., theinput port 520 and the add ports) transmitting the incoming opticalsignals and the output ports (e.g., the pass-through 530 and the dropports) carrying the outgoing optical signals are positioned in analternating (or interleaved) fashion, whereby interposed between everytwo input ports is an output port, and vice versa. Such an arrangementwarrants that if a spectral channel λ_(l) originating from the inputport 520 is to be routed to a drop port, such as the drop port 540-2, byway of the optical system in the dashed box 510, an add spectral channelwith the same wavelength λ_(l) emerging from an adjacent add port 560-2can be directed into the pass-through port 530 by the same opticalsystem in the dashed box 510. This is due to the fact that the dropspectral channel and the corresponding add spectral channel are routedto their respective destinations by the same channel micromirror in thedashed box 510, as to be shown in further detail in FIG. 5B. (Note: inview of the functionality thus described, the add port 560-2 isintrinsically paired with the drop port 540-2; likewise, the add port560-i is paired with the drop port 540-i, where i=1 through N.)

By way of example, FIG. 5B depicts an exemplary channel micromirror 580(e.g., the channel micromirror 103-i shown in FIG. 1B) in a magnifiedschematic view. A first incident beam 581 represents the spectralchannel λ_(l) originating from the input port 520 in FIG. 5A, and afirst reflected beam 582 represents the reflected spectral channel λ_(l)from the channel micromirror 580. A second incident beam 583 representsthe add spectral channel λ_(j) coming from the add port 560-2 in FIG.5A, and a second reflected beam 584 represents the reflected add channelλ_(l) from the channel micromirror 580. Line 585 indicates the normaldirection to the reflective surface of the channel micromirror 580.Because the angle of incidence θ₁ for the first incident beam 581 isequal to the angle of reflection θ₁′ for the first reflected beam 582,and the angle of incidence θ₂ for the second incident beam 583 islikewise the same as the angle of reflection θ₂′ for the secondreflected beam 584, the spatial arrangement by which the input,pass-through, drop, and add ports are positioned in FIG. 5A enables thefirst reflected beam 582 to be routed into the drop port 540-2 and thesecond reflected beam 584 to be directed into the pass-through port 530.

Moreover, by undergoing pivoting (or rotational) motion, e.g., about anaxis 590 (e.g., along the x-direction shown in FIG. 1B) perpendicular tothe plane of the paper, the channel micromirror 580 is further able todirect the spectral channel λ_(l) into any other drop port in theinput-output-port array 570, along with routing an add spectral channel(with the same wavelength) from a pairing add port to the pass-throughport 530, as governed by the same operation principle illustrated inFIG. 5B. The micromirror 580 may also direct the spectral channel λ_(l)to the pass-through port 530, as described in the embodiment of FIG. 1A,2A, 2B, or 3.

One skilled in the art will appreciate that the exemplary embodiment ofFIG. 5B and the operation principle thus illustrated are applicable toany channel micromirror in the OADM 500. As such, the OADM 500 iscapable of inserting (or “adding”) add spectral channels from multipleadd ports to the pass-through port 530, while simultaneously routing thespectral channels from the input port 520 into the appropriate dropports, thereby performing both add and drop functions in a dynamicallyreconfigurable way.

It should be noted that the exemplary embodiment of FIG. 5A is provided,by way of example, to illustrate the general principles of the presentinvention. Various elements and features are shown for illustrativepurposes only, and therefore not drawn to scale. For instance, theinput, pass-through, drop and add ports in the input-output-port array570 are generally not evenly spaced, although they may be approximatelyevenly spaced in a paraxial approximation. From the teachings of thepresent invention, those skilled in the art will also appreciate thatthere are many alternative ways of arranging the input ports and outputports in the input-output-port array 570 of FIG. 5A that would achievesubstantially the same functionality as described above. By way ofexample, the input port 520 and the pass-through port 530 may be placednear the middle of, at the bottom of, or at any other desired locationalong the input-output-port array, so long as the drop ports and addports are accordingly arranged so that interposed between every twoinput ports is an output port, and vice versa. This enables each channelmicromirror to perform a dual function of “dropping” a spectral channelfrom the input port to a drop port and “adding” an add spectral channel(with the same wavelength) from a pairing add port to the pass-throughport, in a manner as depicted in FIG. 5B. Furthermore, theinput-output-port array 570 may be embodied by an array of fibercollimators, which may be conveniently mounted in a V-groove fabricatedon a substrate made of silicon, plastic, or ceramic, as commonlypracticed in the art.

The underlying principle and operation of the embodiments of FIGS. 5A-5Bmay be readily extended to devise an OADM equipped with atwo-dimensional array of input and output ports, as illustrated in FIGS.6A-6B. Shown in FIG. 6A is a schematic top view of a second embodimentof an OADM according to the present invention. By way of example, theOADM 600 makes use of the general architecture of and hence a number ofelements used in the embodiment of FIG. 1A, as identified by thoseelements labeled with identical numerals. In addition, a two-dimensionalinput-output-port array 670 is implemented (where only a top view of theinput-output-port array 670 is explicitly shown), in lieu of the fibercollimator array 110 in FIG. 1A.

FIG 6B depicts a schematic front view of the input-output-port array 670of FIG. 6A. By way of example, the input-output-port array 670 comprisesan input-port column 671 having an input port 620 and a plurality of addports 660-1 through 660-N, and an output-port column 672 including apass-through port 630 and a plurality of drop ports 640-1 through 640-N.In this arrangement, each input port (e.g., add port 660-i) forms a“pair” with its adjacent output port (e.g., drop port 640-i, where i=1through N), in such a way to enable each channel micromirror to route aspectral channel from the input port 620 to a drop port (e.g., the dropport 640-i) and direct an add spectral channel (with the samewavelength) from a pairing add port (e.g., the add port 660-i) to thepass-through port 630, as the arrows in the figure indicate.

In operation, the input port 620 transmits a multi-wavelength opticalsignal. The wavelength-separator in the form of the diffraction grating101 separates the multi-wavelength optical signal by wavelengthrespectively into multiple “incoming spectral channels”. (Note: the“incoming spectral channels” herein refer to the spectral channelsoriginating from the input port 620, in contrast with the add spectralchannels to be described later). The beam-focuser in the form of thefocusing lens 102 focuses the incoming spectral channels intocorresponding focused spots, impinging onto the channel micromirrors103. As a way of example, each channel micromirror may be configuredsuch that in a “nominal position”, it reflects the correspondingincoming spectral channel to the pass-through port 630, as illustratedin FIG. 6A. Furthermore, the diffraction grating 101, along with thefocusing lens 102, may direct add spectral channels emerging from theadd ports in the input-output-port array 670 onto corresponding ones ofthe channel micromirrors 103. By pivoting each channel micromirror aboutan approximate axis (e.g., along the x-direction shown in FIG. 1B), thechannel micromirror is further able to direct the incoming spectralchannel into a drop port (e.g., the drop port 640-i) in theinput-output-port array 670 and an impinging add spectral channel from apairing add port (e.g., the add port 660-i) to the pass-through port630, as illustrated in FIG. 6B.

One skilled in the art will recognize that the input-output-port array670 may be alternatively implemented in any other WSR apparatusaccording to the present invention (e.g., by appropriately modifying theembodiment of FIG. 2A, 2B, or 3). Those skilled in the art will alsoappreciate that the exemplary embodiment of FIGS. 6A-6B is provided, byway of example, to illustrate the general principles of the presentinvention. Various elements and features are shown for illustrativepurposes only, and therefore not drawn to scale. For instance, the inputport 620 and the pass-through port 630 as a pair may be alternativelyplaced near the middle of, at the bottom of, or at any other desiredlocation along the input-output-port array 670, so long as the inputports and the output ports are separately grouped in two columns andpaired accordingly. The input-output-port array 670 may also be embodiedby two columns of fiber collimators, wherein the input-port andoutput-port columns may be mounted in two V-grooves on a substrate, forexample. From the teachings of the present invention, a skilled artisanwill know how to implement various input and output ports in an OADMaccording to the present invention, to best suit a given application.

Furthermore, a two-dimensional array of collimator-alignment mirrors(along with first and second arrays of imaging lenses) may beadditionally implemented between the input-output-port array 670 and thediffraction grating 101 in the embodiment of FIG. 6A (e.g., in a manneras illustrated in FIG. 3), such that each collimator-alignment mirrorcorresponds to either an input port or an output port. Thecollimator-alignment mirror array may be used for controlling thealignment of the multi-wavelength optical signal from the input port andthe add spectral channels from the add ports, as well as for directingthe reflected spectral channels into the output ports.

As such, a notable advantage of the aforementioned OADMs is the abilityto perform both add and drop functions in a dynamically reconfigurablefashion, without involving additional components such as opticalcirculators and/or optical combiners.

A servo-control assembly may be further incorporated in an OADM of thepresent invention, for monitoring and controlling the optical powerlevels of the spectral channels coupled into the output ports. By way ofexample, FIG. 7 depicts how a servo-control assembly 790 may beintegrated in an OADM 710, according to the present invention. The OADM710 may be substantially identical to the embodiment of FIG. 5A, 6A, orany other embodiment according to the present invention. For purpose ofsimplicity and clarity, only the output ports of the OADM 710 areexplicitly shown, including a pass-through port 730 and a plurality ofdrop ports 740-1 through 740-N. (For example, these output ports may bethe pass-through port 530 and the drop ports 540-1 through 540-N of FIG.5A, or the constituents of the output-port column 672 in FIG. 6B). Theservo-control assembly 790 may include a spectral power monitor 791 anda processing unit 792. The spectral power monitor 791 may be opticallycoupled to the pass-through port 730, so as to monitor the optical powerlevels of the spectral channels coupled into the pass-through port 730.The coupling of the spectral power monitor 791 to the pass-through port730 may be accomplished by way of a fiber-optic coupler 730-C via anoptical switch 795, for instance. The processing unit 792, incommunication with the spectral power monitor 791 and the channelmicromirrors in the OADM 710, uses the optical power measurements fromthe spectral power micromirror 791 to provide feedback control of thechannel micromirrors on an individual basis, so as to maintain theoptical power levels of the spectral channels coupled into thepass-through port 730 at desired values. For example, the optical powerlevels of the spectral channels in the pass-through port 730 may beequalized at a predetermined value, as might be desired in an opticalnetworking application. (Note: if the OADM 710 also includes thecollimator-alignment mirrors as depicted in FIG. 2A, 2B, or 3, theprocessing unit 792 may additionally provide control of thecollimator-alignment mirrors, in a manner as illustrated in FIG. 4B.)

In the embodiment of FIG. 7, the spectral power monitor 791 mayadditionally measure the optical power levels of the spectral channelsin the drop ports 740-1 through 740-N, if so desired in a practicalapplication. This may be accomplished by using fiber-optic couplers740-1-C through 740-N-C to “tap off” predetermined fractions of theoptical signals in the drop ports 740-1 through 740-N respectively. Thetapped-off optical signals may be combined by an optical combiner 796,whose output may in turn be coupled to the optical switch 795. Theconfiguration may be such that during normal operation, the opticalswitch 795 is set in a first switching state (1) that allows the opticalsignal diverted from the pass-through port 730 to pass into the spectralpower monitor 791 (while blocking off the combined optical signaldiverted from the drop ports), thereby enabling the optical power levelsof the spectral channels in the pass-through port 730 to be monitoredand further controlled (e.g., equalized). On an occasional or regularbasis, the optical switch 795 may be set in a second switching state (2)that allows the combined optical signal tapped off from the drop portsto pass into the spectral power monitor 791 (while blocking off theoptical signal diverted from the pass-through port 730), therebyallowing the optical power levels of the drop spectral channels to bemeasured. Note that the processing unit 792 need not provide feedbackcontrol of the channels micromirrors in the OADM 710, while the opticalpower levels of the drop spectral channels are being measured.

One skilled in the art will recognize that rather than operating thespectral power monitor 791 in a time-division-multiplexed fashion by wayof the optical switch 795, an auxiliary spectral power monitor may beadditionally employed in the embodiments of FIG. 7, dedicated formonitoring the optical power levels of the spectral channels in the dropports. (The optical switch 795 need not be utilized, in this case.) Ineither scenario, the processing unit, the spectral power monitor(s) andthe fiber-optic couplers of FIG. 7 may be substantially similar to thosedescribed in the embodiment of FIG. 4A (or 4B) in configuration andoperation. The optical combiner 796 may be an N×1 fiber-optic coupler,or any other suitable optical combining means known in the art. Theoptical switch 795 may be a 2×1 switch. Those skilled in the art willappreciate that in lieu of the combination of the optical combiner 796and the optical switch 795, an (N+1)×1 optical switch may bealternatively implemented in FIG. 7, where the (N+1) input ends of theswitch may be coupled to the pass-through port 730 and the drop ports740-1 through 740-N, respectively, and the output end of the switch maybe coupled to the spectral power monitor 791.

It will be appreciated by one skilled in the art that the embodiments ofFIGS. 5A and 6A provide only two of many embodiments of a dynamicallyreconfigurable OADM according to the present invention. Various changes,substitutions, and alternations can be made herein without departingfrom the principles and the scope of the invention as defined in theappended claims. Accordingly, a skilled artisan can design an OADM inaccordance with the principles of the present invention, to best suit agiven application.

Although the present invention and its advantages have been described indetail, the scope of the present invention should be determined by thefollowing claims and their legal equivalents.

1. An optical apparatus comprising: a) a one-dimensionalinput-output-port array, providing multiple input ports including aninput port for a multi-wavelength optical signal and a plurality of addports, and multiple output ports including a pass-through port and aplurality of drop ports, wherein each drop port has a pairing add port;b) a wavelength-separator, for separating said multi-wavelength opticalsignal by wavelength respectively into incoming spectral channels; c) abeam-focuser, for focusing said incoming spectral channels intocorresponding focused spots; and d) an array of channel micromirrorspositioned such that each channel micromirror receives a unique one ofsaid incoming spectral channels, said channel micromirrors beingindividually and continuously pivotable to reflect said incomingspectral channels into selected ones of said output ports; wherein saidwavelength-separator and said beam-focuser further direct one or moreadd spectral channels from said add ports onto corresponding ones ofsaid channel micromirrors, and wherein said input-output-port array isconfigured such that each channel micromirror is able to reflect anincoming spectral channel to a drop port and an impinging add spectralchannel from a pairing add port to said pass-through port.
 2. Theoptical apparatus of claim 1 further comprising a servo-controlassembly, including a spectral power monitor for monitoring opticalpower levels of said reflected spectral channels in said pass-throughport, and a processing unit responsive to said optical power levels forproviding control of said channel micromirrors.
 3. The optical apparatusof claim 2 wherein said optical power levels are maintained at apredetermined value.
 4. The optical apparatus of claim 2 furthercomprising an optical switch having first and second switching states,whereby in said first switching state said spectral power monitor is inoptical communication with said pass-through port, and in said secondswitching state said spectral power monitor is in optical communicationwith said drop ports.
 5. The optical apparatus of claim 2 furthercomprising an auxiliary spectral power monitor, for measuring opticalpower levels of said reflected spectral channels coupled into said dropports.
 6. The optical apparatus of claim 1 further comprising an arrayof collimator-alignment mirrors, in optical communication with saidwavelength-separator and said input-output-port array, for adjustingalignment of said multi-wavelength optical signal from said input portand said add spectral channels from said add ports, and for directingsaid reflected spectral channels into said output ports.
 7. The opticalapparatus of claim 6 further comprising first and second arrays ofimaging lenses, in a telecentric arrangement with saidcollimator-alignment mirrors and said input-output-port array.
 8. Theoptical apparatus of claim 6 wherein each collimator-alignment mirror isrotatable about at least one axis.
 9. The optical apparatus of claim 1wherein each channel micromirror is pivotable about at least one axis.10. The optical apparatus of claim 1 wherein each channel micromirror isa silicon micromachined mirror.
 11. The optical apparatus of claim 1wherein said input-output-port array comprises alternating input andoutput ports.
 12. The optical apparatus of claim 1 wherein saidinput-output-port array comprises fiber collimators.
 13. The opticalapparatus of claim 12 wherein said fiber collimators are mounted in aV-groove on a substrate.
 14. The optical apparatus of claim 1 whereinsaid beam-focuser comprises a focusing lens, and wherein saidwavelength-separator and said channel micromirrors are placedrespectively in first and second focal planes of said focusing lens. 15.The optical apparatus of claim 1 wherein said wavelength-separatorcomprises an element selected from the group consisting of ruleddiffraction gratings, holographic diffraction gratings, echellegratings, curved diffraction gratings, transmission gratings, anddispersing prisms.
 16. The optical apparatus of claim 1 furthercomprising a quarter-wave plate optically interposed between saidwavelength-separator and said channel micromirrors.
 17. The opticalapparatus of claim 16 wherein said quarter-wave plate is opticallyinterposed between said wavelength-separator and said beam-focuser. 18.The optical apparatus of claim 1 wherein said beam-focuser comprises anassembly of lenses.
 19. An optical apparatus comprising: a) aninput-output-port array, providing a plurality of input ports includingan input port for a multi-wavelength optical signal and at least one addport, and a plurality of output ports including a pass-through port andat least one drop port, wherein each drop port has a pairing add port;b) a wavelength-separator, for separating said multi-wavelength opticalsignal by wavelength respectively into incoming spectral channels; c) abeam-focuser, for focusing said incoming spectral channels intocorresponding focused spots; d) an array of channel micromirrorspositioned such that each channel micromirror receives a unique one ofsaid incoming spectral channels, said channel micromirrors beingindividually controllable to reflect said incoming spectral channelsinto selected ones of said output ports; and e) a servo-controlassembly; wherein said wavelength-separator and said beam-focuserfurther direct at least one add spectral channel from said at least oneadd port onto said channel micromirrors, wherein said input-output-portarray is configured such that each channel micromirror is able toreflect an incoming spectral channel to a drop port and an impinging addspectral channel from a pairing add port to said pass-through port, andwherein said servo-control assembly maintains a predetermined couplingof each reflected spectral channel into said pass-through port.
 20. Theoptical apparatus of claim 19 wherein said servo-control assemblyincludes a spectral power monitor for monitoring optical power levels ofsaid reflected spectral channels coupled into said pass-through port,and a processing unit responsive to said optical power levels forproviding control of said channel micromirrors.
 21. The opticalapparatus of claim 20 wherein said optical power levels are maintainedat a predetermined value.
 22. The optical apparatus of claim 19 furthercomprising an optical combiner coupled to said drop ports, and anoptical switch coupled to said optical combiner, said pass-through port,and said spectral power monitor, whereby in a first switching state ofsaid optical switch said spectral power monitor is in opticalcommunication with said pass-through port, and in a second switchingstate of said optical switch said spectral power monitor is in opticalcommunication with said drop ports via said optical combiner.
 23. Theoptical apparatus of claim 19 further comprising an auxiliary spectralpower monitor, for measuring optical power levels of said reflectedspectral channels into said drop ports.
 24. The optical apparatus ofclaim 19 further comprising an array of collimator-alignment mirrors, inoptical communication with said wavelength-separator and saidinput-output-port array, for adjusting alignment of saidmulti-wavelength optical signal from said input port and said addspectral channels from said add ports, and for directing said reflectedspectral channels into said output ports.
 25. The optical apparatus ofclaim 24 further comprising first and second arrays of imaging lenses,in a telecentric arrangement with said collimator-alignment mirrors andsaid input-output-port array.
 26. The optical apparatus of claim 24wherein each collimator-alignment mirror is rotatable about at least oneaxis.
 27. The optical apparatus of claim 19 wherein each channelmicromirror is continuously pivotable about at least one axis.
 28. Theoptical apparatus of claim 19 wherein each channel micromirror is asilicon micromachined mirror.
 29. The optical apparatus of claim 19wherein said input-output-port array is a one-dimensional array, havingalternating input and output ports.
 30. The optical apparatus of claim19 wherein said input-output-port array is a two-dimensional array,configured such that said input ports and said output ports areseparately grouped in two columns.
 31. The optical apparatus of claim 19wherein said input-output-port array comprises fiber collimators. 32.The optical apparatus of claim 19 wherein said beam-focuser comprises afocusing lens having first and second focal planes, and wherein saidwavelength-separator and said channel micromirrors are placedrespectively in said first and second focal planes.
 33. The opticalapparatus of claim 19 wherein said wavelength-separator comprises anelement selected from the group consisting of ruled diffractiongratings, holographic diffraction gratings, echelle gratings, curveddiffraction gratings, transmission gratings, and dispersing prisms. 34.The optical apparatus of claim 19 further comprising a quarter-waveplate optically interposed between said wavelength-separator and saidchannel micromirrors.
 35. The optical apparatus of claim 19 wherein saidbeam-focuser comprises an assembly of lenses.